This invention relates to a timing system to control the inflow or outflow of a fluid from one or more cylinders of an engine and/or an operating machine by means of corresponding valves controlled by means of a camshaft. In particular this invention relates to an overhead camshaft timing system for an internal combustion engine of the type having several valves per cylinder, for example of the five valve type.
It is known that in order to improve the volumetric efficiency of internal combustion engines, in particular the high performance engines used to equip sports cars or GT cars, there is a common design tendency to provide timing systems which instead of the conventional two valves per cylinder (one inlet and one exhaust) provide systems with several valves per cylinder, for example four (two inlet and two exhaust) or five (three inlet and two exhaust) or six (three inlet and three exhaust). In particular the latter two types of arrangement give rise to considerable structural complications in controlling the valves and/or give rise to serious problems with the arrangement of the valves in order to avoid interference, especially between the tappets of the three inlet valves. In order to overcome these problems it is known from U.S. Pat. No. 4,615,309 to use a divaricated arrangement of the inlet valves, placing the intermediate valve of the three obliquely with respect to the two lateral valves in such a way that the three valves can be controlled by a single camshaft, and using tappets having cups of less than normal diameter and, in the case in point, less than the diameter of the cups of the exhaust valves.
Such an arrangement is not however without disadvantages: in particular the use of cups of smaller diameter, especially with valves which are inclined with respect to the camshaft, and therefore subject to possible lateral thrust on the tappets, involves the risk of increasing the contact pressure between the cups and the guides when the latter are mounted in the cylinder head with a consequent risk of breakage of the lubricating oil film and/or in any event greater wear under load of the moving parts.